High-temperature crystal chemistry of layered calcium borosilicates: CaBSiO4(OH) (datolite), Ca4B5Si3O15(OH)5 (‘bakerite’) and Ca2B2SiO7 (synthetic analogue of okayamalite)

• Published in: Physics and chemistry of minerals 2018-05, Vol.45 (5), p.463-473
• Main Authors: Krzhizhanovskaya, Maria G., Gorelova, L. A., Bubnova, R. S., Pekov, I. V., Krivovichev, S. V.
• Format: Article
• Language: English
• Subjects: Anisotropy; Calcium; Calorimetry; Cristobalite; Crystal structure; Crystallography and Scattering Methods; Crystals; Decomposition; Deformation mechanisms; Earth and Environmental Science; Earth Sciences; Geochemistry; High temperature; Melts; Mineral Resources; Mineralogy; Organic chemistry; Original Paper; Silicon dioxide; Thermal expansion; Thermogravimetry; Topology; Wollastonite; X-ray diffraction
• ISSN: 0342-1791; 1432-2021
• DOI: 10.1007/s00269-017-0933-y

The high-temperature behaviour of three Ca borosilicates has been studied by in situ powder high-temperature X-ray diffraction (HTXRD), differential scanning calorimetry and thermogravimetry in the temperature range 30–900 °C for natural samples of datolite, CaBSiO 4 (OH), and ‘bakerite’, Ca 4 B 5 Si 3 O 15 (OH) 5 , and a synthetic analogue of okayamalite, Ca 2 B 2 SiO 7 . The latter was obtained by heating datolite at 800 °C for 5 h. Datolite and bakerite start to dehydroxylate above 700 and 500 °C, respectively, and decompose fully to form a high-temperature modification of okayamalite, HT-Ca 2 B 2 SiO 7 , and wollastonite, CaSiO 3 at about 730 °С. Above 900 °C, HT-okayamalite decomposes with the formation of wollastonite, CaSiO 3 , and metaborate CaB 2 O 4 . The latter melts at about 990 °C. Above 1000 °C, only the existence of wollastonite, CaSiO 3 and cristobalite, SiO 2 was observed. According to the HTXRD data, in the temperature range 30–500 °C, datolite and ‘bakerite’ demonstrate very similar and relatively low volumetric thermal expansion: α v  = 29 and 27 × 10 −6  °C −1 , respectively. A high thermal expansion anisotropy ( α max / α min  ~ 3) is caused by both the layered character of the crystal structures and the shear deformations of their monoclinic unit cells. The direction of maximum expansion is intermediate between the normal direction to the layers and the ( a  +  c ) vector. A possible transformation mechanism from the datolite to the okayamalite structure topology is proposed from geometrical considerations. The synthetic analogue of okayamalite, Ca 2 B 2 SiO 7 , undergoes a reversible polymorphic transition at about 550 °C with a decrease in symmetry from tetragonal to orthorhombic. The crystal structure of the high-temperature (HT) modification of okayamalite was solved from the powder-diffraction data [900 °C: P 2 1 2 1 2, a  = 7.3361(4), b  = 7.1987(4), c  = 4.8619(4) Å, V  = 256.76(3) Å 3 , R wp = 6.61, R Bragg = 2.68%].